High-pressure pumping systems are known for delivering liquid at high pressure. Such a system is described in U.S. Pat. No. 4,883,409 ("the '409 patent"). The '409 patent describes a pumping apparatus for delivering liquid at a high pressure, such as for high performance liquid chromatography ("HPLC") applications. The pumping apparatus comprises two pistons which reciprocate in respective pump chambers. The pistons and pump chambers are connected "serially" in that the output of the first pump chamber is connected via a valve to the input of the second pump chamber. The pistons are driven by linear drives, e.g., ball-screw spindles, and are synchronized so that a first or primary pump head receives its fluid intake at atmospheric or ambient pressure and compresses the intake, or puts it under pressure to a point, just prior to delivering the fluid to the second or accumulator pump head which has a high pressure interconnection with the primary pump head and virtually always receives pressurized fluid.
In the apparatus of the '409 patent, the stroke volume displaced by the respective piston is freely adjustable during a controlled stroke cycle. Control circuitry is operative to reduce stroke volume at reduced flow rates, leading to reduced pulsations in the outflow of the pumping apparatus. According to the '409 patent, the pumping system includes a control means and mechanisms to vary stroke length or volume, and stroke frequency. The control means is operative to adjust the stroke lengths of the pistons between their top dead center and their bottom dead center, respectively, permitting an adjustment of the amounts of liquid displaced by the first and second piston, respectively, during a pump cycle such that pulsations in the flow of the liquid delivered to the output of the pumping apparatus are reduced.
While pulsations at the output are reduced according to the '409 patent, no consideration is given to the presence of gas in the liquid stream. It is acknowledged in the '409 patent that the compressibility of solvents used in HPLC can be problematic, presenting a source of output flow pulsations. However, there is no consideration of the affects of gas in the solvent(s), and the negative implications that gas, i.e. in the form of bubbles, will have on the output of the pumping system and ultimately on the reliability of the chromatograph.
At least one system known in the art identifies problems and includes mechanisms that attempt to address the problems associated with gas in the liquid stream. U.S. Pat. No. 5,393,434 ("the '434 patent") discloses that gas liberated due to reduced pressures during the inlet phase of operation of a pressurized pumping system can accumulate in the pumping chamber and will not be expelled through the outlet because of the back pressure present. Consequently, the pump will stop pumping liquid when the trapped gas remains in the system. Other problems are produced by typical hard seat check valves which can be propped open by particulate matter causing leaks. Also, ordinary inlet valves in known systems are opened on an inlet stroke by suction, which contributes to undesirable gas generation from the liquid being pumped.
According to the '434 patent, a liquid chromatography system is disclosed including a liquid pump having a pumping chamber, an inlet port, an outlet port, and a purge port, all communicating with the pumping chamber. A purge valve is connected to the purge port and is used to purge gas from the system. A disclosed method of operation of the system includes monitoring the pumping performance of the liquid pump to detect the presence of air in the pumping chamber; opening the purge valve; and producing a forward stroke of the piston to discharge the detected air through the purge valve. It is asserted in the '434 patent that purging of the pumping chamber will quickly correct faulty pump performance resulting from air trapped in the liquid phase. The pumping performance is monitored by monitoring the pressure in the pumping chamber, as it is asserted that pumping chamber pressure can indicate the presence of trapped air.
In the parallel, dual pumping implementation of the '434 patent, each liquid pump has a pumping chamber, an inlet valve for receiving liquid, an outlet valve for discharging liquid to a separation column, a piston for drawing liquid through the inlet valve during a backstroke and for discharging liquid through the outlet valve during a forward stroke, and a pressure sensor for sensing the pressure in the pumping chamber. The method of operating such an apparatus involves monitoring the pressure in the pumping chamber with the pressure sensor during the forward stroke of the piston to detect the presence of air in the pumping chamber; determining the deficiency in liquid flow produced by the pump because of the detected air in the pumping chamber; and adjusting the operation of the pump to compensate for the deficiency.
Adjusting pump operation effects desired pump performance by compensating the length of the pump's forward stroke. The adjusting step may include adjusting the speed of the forward stroke of the piston, or adjusting the speed of the backstroke of the piston. In order to effect such a method, the monitoring is performed during an early portion of the forward stroke. Early stroke monitoring facilitates the desired adjustment of pump operation.
In the dual, parallel pump configuration of the '434 patent, monitoring is effected with a first pressure sensor which monitors the pressure in the first pumping chamber to detect an end of the forward stroke by the first piston. Forward stroke of the second piston is initiated in response to the monitoring of the pressure in the first pumping chamber. A second pressure sensor senses the pressure in the second pumping chamber to detect an end of the forward stroke by the second piston. The forward stroke of the first piston follows in response to the sensing of the pressure in the second pumping chamber. Accordingly, controlled parallel pump operation is effected.
Uniform system pressure in the parallel implementation is effected by determining system pressure in the separation system and accordingly initiating the forward stroke of the first piston to provide the system pressure in the first pumping chamber at the end of the forward stroke by the second piston. The forward stroke of the second piston is initiated, at the end of the forward stroke of the first piston, to provide the system pressure in the second pumping chamber. The forward stroke of the second piston is initiated at the end of the forward stroke of the first piston, and the forward stroke of the first piston is initiated at the end of the forward stroke of the second piston. This synchronizes operation of the parallel pump.
Parallel pumps, such as disclosed in the '434 patent have inherent disadvantages. Parallel pump configurations, which by definition alternate delivery between pump heads, tend to have higher levels of unswept volumes. Dead or unswept volumes remain undelivered, and during gradient operation the unswept volume is delivered out of order, i.e. after delivery of the alternate pump head volume, resulting in compositional ripple and/or inaccurate chromatographic peaks.
Furthermore, the mechanism effected in the '434 patent disadvantageously includes a spring loaded outlet check valve which requires additional mechanical parts to address problems associated with gas in the liquid stream. The outlet check valve prevents fluid passage from the pump outlet to a pulse dampener when gas is trapped in the pump chamber(s). To prevent fluid flow from stopping altogether, a separate purge valve is activated to facilitate escape of the gas. When a large drop in pressure is sensed by the pressure transducers, it is assumed that there is gas in the pump chamber. At the onset of the pressure drop, the purge valve is opened, i.e. turned on, and the gas bubble is expelled. No record is maintained of the expulsion of the gas and there is no mechanism to cross-check gas expulsion against particular chromatographic runs to flag potentially erroneous runs. A fairly high degree of solvent conditioning at the input is required to avoid excessive opening of the check valves which can have a detrimental impact on efficacy of the system. Moreover, the '434 patent parallel design requires two additional check valves and two additional purge valves, with each being comprised of six or more additional moving parts. These parts represent additional cost. Long term performance and reliability of all of these additional parts is difficult to maintain.
In addition to the fact that the added mechanisms, in the form of the check valves and purge valves, represent unnecessary mechanical complexity and cost in the system according to the '434 patent, the check valves, as discussed in the '434 patent, present an opportunity for gas to enter the system and/or for leaks to develop. Failure of the mechanical check valves to expel gas from the system can result in the loss of prime of the pumps which will shut the system down. The purge valve and inlet check valve have unswept volumes or flow areas which will disadvantageously contribute to band spreading or broadening of chromatographic peaks. The increased volume in the pump heads due to check valves and purge valves leads to lower compression ratios for pumps according to the '434 patent design, which increases the difficulty in expelling bubbles.